1. Field of the Invention
The present invention relates to a light emitting device, and particularly, relates to a surface light emitting device and a polarized light source, which emit polarized light.
2. Description of the Related Art
In recent years, a light emitting element that emits polarized light has been developed. A polarized light source including such a light emitting element is used as a light source of a liquid crystal display or a front-type projector, whereby a light utilization efficiency of the liquid crystal display or the front-type projector can be enhanced. A reason for this is as follows. Specifically, such a display device as the liquid crystal display includes a pair of polarizing plates arranged so as to sandwich a liquid crystal cell therebetween, and accordingly, if the polarized light is emitted from the light emitting device, then a light component that can pass through the polarizing plate located on a front side of the liquid crystal cell is increased, and hence, a quantity of light that can pass through the liquid crystal cell is increased.
The above is described below with reference to FIG. 1. As shown in FIG. 1, as a surface light emitting device 110 of a liquid crystal display 100, there is proposed a backlight including: a polarized light source 111 that emits polarized light 150; and a resin-made light guide plate 112 that receives the polarized light 150 emitted from the polarized light source 111, disperses the received polarized light 150 in an inside thereof, and uniformly emits light 160 from a light emitting surface 112a. In FIG. 1, polarizing plates 120 and 140 are polarizing plates arranged to sandwich a liquid crystal cell 130 therebetween.
In usual, the resin-made light guide plate 112 shown in FIG. 1 is manufactured by molding resin such as acrylic resin into a predetermined shape by a method such as injection molding. At the time of molding the resin, orientation of molecules occurs in the resin. Moreover, also in the case of thermally curing the resin after the molding, the orientation occurs in the resin molecules, and a residual stress is generated in the light guide plate. The occurrence of the orientation of the resin molecules and the generation of the residual stress allow appearance of birefringence in the inside of the light guide plate. When the birefringence appears in the resin composing the light guide plate, polarization characteristics are disturbed by the fact that the polarized light generated in the polarized light source passes through the light guide plate. As a result, the polarized light nearly becomes random polarized light. Hence, in actual, the light 160 radiated from the light guide plate 112 shown in FIG. 1 cannot maintain the polarization characteristics, and an advantage brought by using the polarized light source 111 that emits the polarized light 150 cannot be fully utilized. Incidentally, in the case of transmitting the random polarized light through the polarizing plate, intensity of light that has passed through the polarizing plate is attenuated to approximately 35%.
Besides the above-described problem in the light guide plate 112, a similar problem is inherent in the polarized light source 111 as a package. Specifically, in a similar way to the manufacture of the light guide plate 112, also in the process of manufacturing the polarized light source 111, at the time of thermally curing sealing resin, such as epoxy resin and silicon resin, that seals a light emitting element (not shown), the orientation occurs in molecules of the sealing resin, and the residual stress is generated in the sealing resin. The occurrence of the orientation of the resin molecules and the generation of the residual stress allow appearance of the birefringence in an inside of the sealing resin. When the birefringence appears in the sealing resin, the polarization characteristics are disturbed by the fact that the polarized light generated in the light emitting element passes. through the sealing resin. Hence, the polarization characteristics of the polarized light radiated from the polarized light source 111 as a package cannot be maintained, and the advantage brought by using the light emitting element that emits the polarized light cannot be fully utilized.
As a result, in the light 160 emitted from the light guide plate 112, a light component that cannot pass through the polarizing plate 120 is increased, and light 170 with a small light quantity passes through the liquid crystal cell 130. Then, a light component that can pass through the polarizing plate 140 as an analyzer is further decreased. Therefore, brightness of display in the liquid crystal display 100 is low, and a contrast thereof is also decreased.